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Abstract:

A method is disclosed. The method includes obtaining a precursor
nanoparticle comprising a base material and a first ligand attached to
the base material, and reacting the precursor nanoparticle with a
reactant comprising a silicon bond, thereby removing the first ligand.

Claims:

1. A method comprising: obtaining a precursor nanoparticle comprising a
base material and a first ligand attached to the base material; and
reacting the precursor nanoparticle with a reactant comprising a silicon
bond, thereby removing the first ligand.

2. The method of claim 1 wherein the base material comprises a core
material and a shell material surrounding the core material.

3. The method of claim 1 wherein the first ligand comprises an
alkylphosphonate ligand, an alkylphosphinate ligand, or an
alkylcarboxylate ligand.

4. The method of claim 1 wherein the reactant comprising silicon is a
compound comprising silicon, wherein the compound comprising silicon
comprises Si--X, wherein X comprises at least one atom selected from the
group consisting of S, Se, and halides.

5. The method of claim 1 wherein reacting the nanoparticle with the
reactant comprising silicon is performed in solution.

6. The method of claim 1 wherein the reactant comprising silicon
comprises silicon and a second ligand, and wherein after reacting, the
second ligand is attached to the base material instead of the first
ligand.

7. A nanoparticle formed by the method of claim 1.

8. An electronic device comprising the nanoparticle of claim 7.

9. A nanoparticle comprising: a base material; and a ligand attached to
the based material, wherein the ligand comprises an atom selected from at
least columns V, VI and VII of the periodic table.

14. A method comprising: obtaining a precursor nanoparticle comprising a
base material and a first ligand attached to the base material; and
reacting the precursor nanoparticle with a reactant comprising a silicon
bond, thereby removing the first ligand, wherein the reactant comprises
silicon and a second ligand, and wherein after reacting, the second
ligand is attached to the base material instead of the first ligand, and
wherein the second ligand includes a halide.

15. The method of claim 14 wherein the base material comprises a core
material and a shell material surrounding the core material.

16. The method of claim 14 wherein the first ligand comprises an
alkylphosphonate ligand, an alkylphosphinate ligand, or an
alkylcarboxylate ligand.

17. The method of claim 14 wherein the reactant comprising silicon is a
compound comprising silicon, wherein the compound comprising silicon
comprises Si--X, wherein X comprises at least one atom selected from the
group consisting of S, Se, and halides.

18. The method of claim 14 wherein reacting the nanoparticle with the
reactant comprising silicon is performed in solution.

19. The method of claim 14 wherein the reactant comprising silicon
comprises silicon and a second ligand, and wherein after reacting, the
second ligand is attached to the base material instead of the first
ligand.

20. A nanoparticle formed by the method of claim 14.

21. An electronic device comprising the nanoparticle of claim 20.

22. A nanoparticle comprising: a base material; and a ligand attached to
the based material, wherein the ligand comprises an atom from column VII
of the periodic table.

Description:

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a non-provisional of and claims the benefit of
the filing date of U.S. Provisional Patent Application No. 61/084,852,
entitled "Chemical Modification of Nanocrystal Surfaces," filed on Jul.
30, 2008, which is herein incorporated by reference in its entirety for
all purposes.

BACKGROUND OF THE INVENTION

[0003] It remains a major challenge to directly determine nanoparticle
surface structures, because of the lack of analytical tools that are
currently available. (Alivisatos, A. P., J. Phys. Chem., 100:13226-13239
(1996)). Early nuclear magnetic resonance (NMR) and X-ray photoelectron
spectroscopy (XPS) studies of CdSe nanocrystals prepared in coordinating
solvents such as tri-n-octylphosphine oxide and tri-n-octylphosphine,
suggested these coordinating solvents are datively bound to the surface
of a nanoparticle. (Bowen-Katari, J. E. et al., J. Phys. Chem., 98:4109
(1994); Becerra, L. R. et al., J. Chem. Phys., 100:3297-3300 (1994)).
However, assigning the broad NMR resonances of surface-bound ligands is
complicated by significant concentrations of phosphorus-containing
impurities in commercial sources of tri-n-octylphosphine oxide (1), and
XPS provides only limited information about the nature of the phosphorus
containing molecules in the sample.

[0005] In general, chemical modification of nanocrystal surfaces using
ligand exchange processes is desirable. For example, nanocrystals with
modified surfaces can be used in biology and medicine. Also, modifying
the surfaces of nanocrystals can change the electrical and optical
properties of such nanocrystals.

[0006] Surface modification of nanocrystals using (--SH) is known. While
effective in some instances, it would be desirable to provide for other
types of ligand reactions, which may be used to produce different types
of nanocrystals. Nanocrystals with different types of ligands could be
advantageously used in various applications (e.g., electronics).

[0007] These and other problems are addressed individually and
collectively by embodiments of the invention.

SUMMARY OF THE INVENTION

[0008] Embodiments of the invention relate to nanoparticles, methods for
making nanoparticles, and devices incorporating nanoparticles.

[0009] One embodiment of the invention is directed to a method comprising
obtaining a precursor nanoparticle comprising a base material and a first
ligand attached to the base material, and reacting the precursor
nanoparticle with a reactant comprising a silicon bond (e.g., Si--X),
thereby removing the first ligand. In some cases, the reactant comprises
a second ligand and a silicon atom, and the second ligand replaces the
first ligand on the base material.

[0010] Another embodiment of the invention is directed to a nanoparticle
comprising: a base material and a ligand attached to the based material,
wherein the ligand comprises at least one atom from columns V, VI and VII
of the periodic table.

[0011] Another embodiment of the invention is directed to a method
comprising: obtaining a precursor nanoparticle comprising a base material
and a first ligand attached to the base material; and reacting the
precursor nanoparticle with a reactant comprising a silicon bond, thereby
removing the first ligand. The reactant comprises silicon and a second
ligand comprising a halide. After reacting the base material and the
reactant, the second ligand is attached to the base material instead of
the first ligand.

[0012] Another embodiment of the invention is directed to a nanoparticle
comprising: a base material; and a ligand attached to the based material,
wherein the ligand comprises an atom from column VII of the periodic
table.

[0013] These and other embodiments of the invention are described in
further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows {1H}31P NMR spectra of 167 mg of as prepared
CdSe nanocrystals in 0.6 ml d8-toluene (left), and the reaction
between CdSe nanocrystals and bis(trimethysilyl)selenide (4) in
d8-toluene.

[0019] The Figures are referenced below in the Detailed Description
section of the present application.

DETAILED DESCRIPTION

[0020] Embodiments of the invention include nanoparticles and methods for
making nanoparticles.

[0021] One embodiment of the invention comprises a method including
obtaining a precursor nanoparticle comprising a base material and a first
ligand (e.g., a phosphonate ligand) attached to the base material (e.g.,
CdSe). Suitable examples of first ligands may include phosphorous
containing ligands (e.g., an alkyl phosphonate ligand or an
alkylphosphinate ligand) or carboxylate containing ligands such as
alkylcarboxylate ligands. Other examples may include carbamate,
carbonate, alkoxide, phosphinate, sulphinate, and sulphonate containing
ligands. In some embodiments, the first ligands may be any suitable
ligands including an anionic group, which can be bonded to a cationic
site on the surface of the precursor nanoparticle.

[0022] The method also includes reacting the precursor nanoparticle with a
reactant comprising a silicon bond (e.g., Si--X), thereby removing at
least the first ligand. The reaction of the precursor nanoparticle and
the reactant comprising the silicon bond can occur in solution.

[0023] In embodiments of the invention, the reactant may comprise a
silicon atom and a second ligand attached to the silicon atom. The second
ligand may be X, which may comprise at least one atom selected from
columns V, VI, and VII of the periodic table. For instance, X may be a
halide such as Cl, Br, or I. In another example, X may be a sulphur
containing ligand such as --SR, where R is a hydrocarbon chain,
--S--SiMe3, --S--(CH2CH2O)4)CH3, etc. X could
alternatively be an Se containing ligand such as --SeSiMe3. The
second ligand may be attached to the base material of the precursor
nanoparticle after the reaction, thereby replacing the first ligand and
forming a processed nanoparticle (e.g. a nanoparticle comprising CdSe
sphere with Cl ligands attached to the CdSe sphere).

[0024] The precursor nanoparticle may comprise any suitable composition or
configuration and can be one or many precursor nanoparticles in solution.
After the above-described reaction, the precursor nanoparticles may be
processed to form processed nanoparticles. The processed nanoparticles
may have the second ligand instead of the first ligand. For example, a
first ligand such as a phosphorous containing group (e.g., an alkyl
phosphonate ligand or an alkylphosphinate ligand) or a carbon containing
group such as an alkylcarboxylate ligand, on a CdSe nanocrystal, may be
replaced with a second ligand comprising Se, which extends the base
material with another layer of Se.

[0025] As used herein, "nanoparticles" can refer to crystalline particles
that have at least one dimension less than about 100 nanometers. In some
embodiments of the invention, the nanoparticles may have two or more
dimensions that are less than about 100 nanometers. The nanoparticles may
be in the form of spheres, rods, tetrapods, branches, etc.

[0026] Nanoparticles may also include branched nanoparticles. Branched
nanoparticles can have arms that have aspect ratios greater than about 1.
In other embodiments, the arms can have aspect ratios greater than about
5, and in some cases, greater than about 10, etc. The widths of the arms
may be less than about 200, 100, and even 50 nanometers in some
embodiments.

[0027] The nanoparticles may comprise semiconductors such as compound
semiconductors. Suitable compound semiconductors include Group II-VI
semiconducting compounds such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS,
HgSe, and HgTe. Other suitable compound semiconductors include Group
III-V semiconductors such as GaAs, GaP, GaAs--P, GaSb, InAs, InP, InSb,
AlAs, AlP, and AlSb. The use of Group IV semiconductors such as germanium
or silicon may also be feasible under certain conditions. Suitable
methods for forming precursor nanoparticles can be found in U.S. Pat.
Nos. 6,225,198 and 6,306,736.

[0028] Any of the foregoing materials may form a core material and/or a
shell material, which surrounds a core material. The core material and/or
the shell material may be part of a "base material." In embodiments of
the invention, the base material is not altered during the reaction with
the reactant containing the silicon bond. Rather, functional groups at
the surface of the base material may react with the reactant containing
the silicon bond.

[0029] Illustratively, the core material may comprise a CdSe sphere, and a
shell material surrounding the CdSe sphere may be a ZnS shell. The
CdSe/ZnS core/shell structure may form a base material. In other
embodiments, the base material need not be in the form of a composite,
but can be formed from a single unitary material such as CdSe.

[0030] After forming the nanoparticles, they may be further processed in
any suitable manner. For example, if the nanoparticles comprising the
base material are not substantially pure, then they may be purified using
any suitable purification process known in the art.

[0031] As noted above, the reactant that is used can comprise any suitable
material including a silicon bond. For example, the reactant can comprise
a compound comprising a silicon bond. One atom of the silicon bond may be
silicon, while the other atom may be at least one atom from columns V,
VI, or VII of the periodic table. In some embodiments, the other atom is
S, Se, Te, or a halide (e.g., Cl, or Br). Examples of suitable reactants
include bis(trimethysilyl)selenide,
S-trimethylsilyl-2,5,8,11-tetraoxatridecane-13-thiol, and Me3Si--X
(X=--S--SiMe3, --Se--SiMe3, --Cl and
--S--(CH2CH2O)4OCH3)).

[0032] After the precursor nanoparticles are formed, they may be mixed in
solution with the reactant. Any suitable solvent may be used as a liquid
medium for mixing the precursor nanoparticles and the reactant. Suitable
solvents include hydrocarbon solvents such as toluene, chloroform,
hexane, diethylether, etc.

[0033] Mixing may also occur in any suitable manner. Conventional mixers
and stirrers may be used to mix the reactant and the precursor
nanoparticles.

[0034] The reaction may proceed at any suitable temperature and pressure
sufficient to induce the removal of the first ligand from the precursor
nanoparticles. Suitable temperatures can be equal to or higher than
ambient temperature, but lower than the boiling point of the solvents
used. For example, the reaction may proceed at about 298 K in some
embodiments.

[0035] Other reagents such as surfactants may also be included in the
reaction solution. Surfactants can help to keep the precursor
nanoparticles and/or subsequently processed nanoparticles in solution.
Examples include tetraalkylammonium halides, N,N'-dialklylimidazolium
halides, alkylamines, alkylphosphines, alkylthiols, etc.

[0036] After mixing and reacting the precursor nanoparticles and the
reactant, processed nanoparticles are formed. The processed nanoparticles
may comprise the base material of the precursor nanoparticles. In some
embodiments, the processed nanoparticles may include the base material
with second ligands attached thereto. The second ligands may be ligands
X, which were previously bonded to silicon atoms in the reactant
comprising the silicon bond. The processed nanoparticles (and also the
precursor nanoparticles) may have ligands that are only of one type
(e.g., all Cl atoms), or may alternatively have mixtures of different
ligands on them (e.g., a mixture of Cl and Br atoms in a monolayer on the
base material each nanoparticle in a cluster of nanoparticles). In other
embodiments, the processed nanoparticles may not have second ligands
attached to the base material. In such embodiments, the first ligands in
the precursor nanoparticles may simply be removed from the base material.

[0037] Advantageously, embodiments of the invention can use a reactant
with a silicon containing bond. The use of this type of reactant is
desirable, since the use of a silicon containing bond is
thermodynamically favorable when removing a first ligand from a base
material of a precursor nanoparticle.

[0038] Also, by using a silicon bond in the reactant, unique ligands such
as ligands containing Se, Te, Cl, Br, etc. can be attached to a base
material of a precursor nanoparticle. Ligands such as Cl, Br, and other
halides are desirable, since they are small and can allow for
nanoparticles containing such surface atoms to be packed closely
together. This can be desirable, for example, in the electronics industry
where the close packing of nanoparticles is desirable to provide for
conduction in an electronic device. For example, the closely packed
nanoparticles can be used as conductive lines in microcircuits, or solar
devices such as solar cells. A microcircuit typically has a layer of
insulating material with conductive lines formed on the insulating
material. In other embodiments, nanoparticles that are closely packed
together could be used to form an insulating layer in an electronic
device. Also, such unique ligands can be used to induce subsequent
reactions with the processed nanoparticles. For example, a nanoparticle
with a chlorine monolayer at its surface may be further processed so that
other ligands may replace the chlorine monolayer. Reactions may also be
possible with a chlorine monolayer.

[0039] The formed nanoparticles according to embodiments of the invention
can have unique optical, electrical, magnetic, catalytic, and mechanical
properties, and can be used in a number of suitable end applications.
They can be used, for example, as fillers in composite materials, as
catalysts, as functional elements in optical devices, as functional
elements in photovoltaic devices (e.g., solar cells), as functional
elements in electrical devices, etc. They can also be used in LEDs, as
biological labels, etc.

EXAMPLES

[0040] A number of examples are provided below. Embodiments of the
invention are not limited to the description of such examples.

[0041] By using reagents with reactive silicon-chalcogen and
silicon-chlorine bonds to cleave the ligands from the nanocrystal
surface, it can be shown that as-prepared CdSe and CdSe/ZnS core-shell
nanocrystal surfaces are likely terminated by X-type binding of
alkylphosphonate ligands to a layer of Cd2+/Zn2+ ions, rather
than by dative interactions. Further, spectroscopic evidence that
tri-n-octylphosphine oxide (1) and tri-n-octylphosphine (2) are not
coordinated to the purified nanocrystals is provided.

[0042] 3-6 nm CdSe nanocrystals were synthesized by reacting
tri-n-octylphosphine selenide with anhydrous
cadmium-n-octadecylphosphonate prepared from dimethylcadmium and
n-octadecylphosphonic acid (3) in tri-n-octylphosphine oxide (1) at
315° C. ZnS shells were grown on these cores by reacting
zinc-n-octadecylphosphonate with bis(trimethylsilyl)sulfide under similar
conditions. Both tri-n-octylphosphine oxide (1) and n-octadecylphosphonic
acid (3) were recrystallized prior to use and shown to be free of
phosphorus-containing impurities with NMR spectroscopy. To ensure the
purity of the nanocrystal product, removal of remaining cadmium- and
zinc-n-octadecylphosphonate, insoluble coordination polymers (Cao, G. et
al., Chem. Mater, 5:1000-1006 (1993)), was accomplished by their
depolymerization and dissolution with octylamine, followed by fractional
precipitation of the nanocrystals.

[0043]1H-NMR spectra of purified nanocrystals in d8-toluene
showed broad resonances for methylene groups (δ=1.3-4.0 ppm) and
methyl groups (δ=0.9-1.0 ppm) in a ratio of ˜17:1
representative of octadecyl chains. Additionally, a broad resonance of
low intensity is visible between δ=7.8-9.2 ppm, which was
tentatively assigned to a low concentration of acidic hydrogens present
in the ligand shell. (Assuming this resonance corresponds to the acidic
hydrogen of an octadecylphosphonic acid ligand bound to the nanocrystal
accounts for only one hydrogen per 11.5±2% of the octadecylphosphonate
moieties.) A {1H}31P-NMR spectra of a concentrated sample (278
mg/mL) showed a broad bimodal resonance from δ=10-40 ppm
reminiscent of the spectrum published by Bawendi and coworkers, and
originally interpreted to be characteristic of surface-bound
tri-n-octylphosphine oxide (1) and tri-n-octylphosphine (2) (FIG. 1).
Neither the 1H nor the {1H}31P-NMR spectrum showed sharp
resonances that might arise from "free" surfactant molecules.

[0044] Removal of these surface-bound ligands was accomplished by adding
bis(trimethysilyl)selenide (4) to a solution of the CdSe nanocrystals in
d8-toluene. Shortly after addition (10-60 minutes), the sample
became turbid and the nanocrystals then slowly settled out of solution.
NMR spectra of these solutions immediately after mixing are dramatically
sharpened due to release of the surface-bound ligands (FIG. 1). In
particular, three sharp resonances characteristic of "free" small
molecules appeared in the {1H}31P-NMR spectrum that were
assigned to O,O'-bis(trimethylsilyl)octadecylphosphonic acid (5) and a
mixture of racemic and meso O,O'-bis(trimethylsilyl)octadecylphosphonic
acid anhydride (6) (FIG. 1). Similar reactivity was observed with
bis-trimethylsilylsulfide. Both mass spectrometry and an independent
synthesis of these reaction byproducts confirmed the assignment.

[0045] The presence of the n-octadecylphosphonic acid anhydride in the
ligand shell likely arises from reaction of n-octadecylphosphonic acid
(3) with tri-n-octylphosphine selenide during the synthesis of CdSe,
rather than as a byproduct of the ligand cleavage reaction. (Liu, H., et
al. J. Am. Chem. Soc., 129:305-312 (2007)). This is further supported by
the observation that increasing amounts of meso
O,O'-bis(trimethylsilyl)octadecylphosphonic acid anhydride (6) relative
to O,O'-bis(trimethylsilyDoctadecylphosphonic acid (5) are cleaved from
nanocrystals synthesized in reactions run to higher conversion of the
cadmium and selenium nanocrystal precursors.

[0046] The reactivity of the silicon-selenium and silicon-sulfur bonds and
the stability of the newly formed silicon-oxygen bond presumably provide
the driving force for this reaction, and led the present inventors to
attempt a similar ligand cleavage with a trimethylsilyl-protected thiol
(7). In this case however, by reacting
S-trimethylsilyl-2,5,8,11-tetraoxatridecane-13-thiol with the
nanocrystals, a thiolate is exchanged for the phosphonates as shown in
Scheme 2 in FIG. 2. Addition of
S-trimethylsilyl-2,5,8,11-tetraoxatridecane-13-thiol (7) to a
d8-toluene solution of the CdSe nanocrystals resulted in rapid
disappearance of the broad 31P-NMR resonance of the starting
material and signals characteristic of
O,O'-bis(trimethylsilyDoctadecylphosphonic acid (5) and a mixture of
racemic and meso O,O'-bis(trimethylsilyl)octadecylphosphonic acid
anhydride (6), as described above, but did not result in nanocrystal
aggregation. The appearance of a broad resonance for the bound thiol in
the 1H NMR spectrum was also evident (δ=3.2-4.5 ppm). Removal
of the solvent en vacuo and addition of anhydrous hexane resulted in
partial dissolution of the nanoparticle product. Isolation of the solids
by centrifugation and washing with anhydrous hexane produced nanocrystals
that are soluble in polar solvents like water, methanol and chloroform
was performed. A 1H NMR spectrum of the newly derivatized
nanocrystals in d8-toluene showed that 85% of the surface ligands
are derived from S-trimethylsilyl-2,5,8,11-tetraoxatridecane-13-thiol (7)
and 15% from octadecyl chains. (The relative integrals of the methylene
and methyl resonances (1:17) from the remaining aliphatic chains showed
that they are composed of octadecyl chains.) Analysis of the
hexane-soluble portion with 1H and {1H}31P-NMR
spectroscopy and ESI-TOF mass spectrometry showed the presence of 5-7 but
no 31P NMR resonances for tri-n-octylphosphine oxide and
tri-n-octylphosphine.

[0047] The exchange the alkylphosphonate ligands for chloride ligands was
also investigated. Adding anhydrous cholortrimethylsilane to a toluene
solution of nanocrystals, results in rapid particle aggregation.
Repeating this experiment in a mixture of toluene saturated with
anhydrous tridecyltrimethylammonium chloride (8), however, prevented
nanocrystal aggregation. (Adding anhydrous tridecyltrimethylammonium
chloride (8) to the nanocrystals resulted in a ˜5 nm shift of the
fluorescence maximum.) Removal of excess anhydrous
tridecyltrimethylammonium chloride (8) by centrifugation and subsequent
fractional precipitation with hexane gave a nanocrystal solid that is
soluble in toluene and chloroform. {1H}31P-NMR spectroscopy of
the reaction byproducts in d8-toluene showed the presence of
O,O'-bis(trimethylsilyl)octadecylphosphonic acid (5) and a mixture of
racemic and meso O,O'-bis(trimethylsilyl)octadecylphosphonic acid
anhydride (6), while a 1H-NMR spectrum of the nanocrystals showed
resonances characteristic of the tridecyltrimethylammonium ion (FIG. 3).
Repeating this experiment with CdSe/ZnS core-shell particles resulted in
a 25% decrease in the fluorescence quantum yield. (All reactions of
chlorotrimethylsilane with nanoparticles were conducted with two
equivalents relative to the number octadecylshains in the sample as
determined by 1H-NMR spectroscopy. Addition of excess
chlorotrimethylsilane results in etching of the CdSe particles, as
evidenced by a blue-shifting of their absorption and fluorescence
spectra, as well as a decrease in quantum yield of the CdSe/ZnS
core-shell samples.). Referring to FIG. 3, the number of chloride ligands
on the particle surface is equal to the number of phoshphonate linkages
transformed to chloride ligands, plus the number of adsorbed chlorides
anions from anhydrous tridecyltrimethylammonium chloride (8), denoted n
in FIG. 3.

[0049] The facile ligand cleavage and exchange reactivity of the
trimethylsilylchalcogenides and chlorotrimethylsilane presented above
suggests that the nanocrystal surfaces may be terminated by X-type
binding of anionic alkylphosphonate moieties to Cd2+ ions on the
crystal surface. This hypothesis is best supported by a model where a
layer of excess cadmium ions bind to the Lewis basic selenide surface
sites of the CdSe core and are charge balanced by the phosphonate ligand.
Previously reported Rutherford backscattering experiments also concluded
that CdSe nanocrystals contain excess Cd ions on their surfaces. (Taylor,
J. et al., J. Clust. Sci., 12:571-582 (2001)). This conclusion, however,
does not strictly follow from these ligand cleavage experiments, since a
control experiment showed free phosphonic acid (3) and its anhydride also
produce O,O'-bis(trimethylsilyl)octadecylphosphonic acid (5) and a
mixture of racemic and meso O,O'-bis(trimethylsilyl)octadecylphosphonic
acid anhydride (6), respectively, on reaction with
bis(trimethysilyl)selenide (4).

[0050] To further investigate the nature of the binding between
n-octadecylphosphonic acid (3) and the nanocrystal surface, the direct
reaction of our CdSe nanocrystals with thiols was studied. Addition of
2-methoxyethanethiol (9) or its long chain counterpart
2,5,8,11-tetraoxatridecane-13-thiol (10) to the CdSe nanocrystal sample
resulted in minimal changes to the resonances for the surface-bound
octadecyl chains in the 1H NMR, though a broad resonance
(δ--3.2-4.5 ppm) appeared upfield of the free thiol. (A small
concentration (<10%) of free surfactant ligands appeared upon the
addition of O,O'-bis(trimethylsilyl)octadecylphosphonic acid (5), that
rapidly reached equilibrium. Surprisingly, heating this sample to
100° C. for 16 hours made little difference to these spectra.)
Repeated precipitation of these nanoparticles from toluene by addition of
hexane furnished a nanoparticle product that retained the broad
signatures of the bound thiol (δ=3.2-4.5 ppm) as well as the
starting octadecylphosphonate ligands in an approximate 1:1 ratio. No
sharp lines indicative of "free" surfactant molecules were visible.
Additionally, the nanocrystal fluorescence was immediately quenched upon
addition of 2-methoxyethanethiol (9) or its long chain counterpart
2,5,8,11-tetraoxatridecane-13-thiol (10). Both observations indicating
that the thiol binds the nanocrystal surface. Repeating this experiment
in the presence of added triethylamine, however, resulted in rapid
(t<10 minutes) sharpening of the aliphatic resonances in the
1H-NMR spectrum. At the same time two sharp resonances in the
31P-NMR spectrum (δ=16.6, 26.1 ppm) appeared that can be
assigned to the conjugate base of octadecylphosphonic acid and
octadecylphosphonic acid anhydride. (The important resonance structures
of the dianionic form of octadecylphosphonic acid anhydride show that
this molecule, unlike the mixture of racemic and meso
O,O'-bis(trimethylsilyl)octadecylphosphonic acid anhydride (6), is not
chiral. Similarly, its protonated form shows a single line spectrum as a
result of rapid hydrogen ion exchange between the P--OH and P═O
functionalities.

[0051] The inability of 2,5,8,11-tetraoxatridecane-13-thiol (10) to
displace the alkylphosphonate ligands in the absence of added base best
supports the conclusion that alkylphosphonate moieties are bound to
cationic cadmium sites as an anion, rather than by a simple dative
interaction. Accordingly, the thiol converts the alkylphosphonate ligand
to an equivalent of free phosphonic acid in order to displace it from the
nanocrystal and form a Cd2+-thiolate interaction. The lack of this
reactivity is likely to arise from a greater pKa of the thiolate ligand,
which remains preferentially protonated over the alkylphosphonate oxygen.
These results indicate that surface exchange reactions ought to be
designed with the need to balance charges between the surface Cd2+
layer and the incoming ligand.

[0053] Further, that free thiols adsorb to our nanocrystal surfaces
without displacing the octadecylphosphonate ligands, and that
chloride-terminated nanocrystals can be made soluble by the addition of
long-chain tetraalkylammonium chloride salts, supports the idea that
there are Lewis-acidic coordination sites in addition to those occupied
by the X-type phosphonate ligands. Integration of our 1H NMR spectra
indicates there are an approximately equal number of L- and X-type sites
available for ligand binding. The presence of these Lewis-acidic
coordination sites may explain the numerous reports that the addition of
dative ligands can change the solubility and optical properties of
cadmium selenide nanocrystals. (See Kalyuzhny, G., Murray, R. W., J.
Phys. Chem. B, 109:7012-7021 (2005) and references cited within.)

[0054] These results shed new light on the chemistry and reactivity of
CdSe nanocrystal surfaces. Preliminary studies suggest similar chemical
reactivity of other II-VI semiconductor nanocrystals. The conclusion that
alkylphosphonate ligands are bound via X-type interaction with Cd2+
can not only influence the development of more powerful ligand exchange
reactions, but will allow for more sophisticated understanding of how
ligands and surfaces control the optical and electrical properties of
nanocrystals. Furthermore, the ability to convert the alkylphosphonate
ligands to other X-type ligands, like chloride, will undoubtedly have a
significant impact on the electrical properties of these nanocrystals.

[0055] In yet another example, referring to FIG. 5, the surface chemistry
of cadmium selenide nanocrystals, prepared from tri-n-octylphosphine
selenide and cadmium octadecylphosphonate in tri-n-octylphosphine oxide,
was studied with 1H and {1H}31P NMR spectroscopy as well
as ESI-MS and XPS. The identity of the surface ligands was inferred from
reaction of nanocrystals with Me3Si--X(X=--S--SiMe3,
--Se--SiMe3, --Cl and --S--(CH2CH2O)4OCH3)) and
unambiguous assignment of the organic byproducts,
O,O'-bis-trimethylsilyloctadecylphosphonic acid ester and
O,O'-bis-trimethylsilylocatdecylphosphonic acid anhydride ester.
Nanocrystals isolated from these reactions have undergone exchange of the
octadecylphosphonate ligands for --X as was shown by 1H NMR
(X=--S--(CH2CH2O)4OCH3) and XPS (X=--Cl). Addition of
free thiols to as prepared nanocrystals results in binding of the thiol
to the particle surface and quenching of the nanocrystal fluorescence.
Isolation of the thiol-ligated nanocrystals shows this chemisorption
proceeds without displacement of the octadecylphosphonate ligands
suggesting the presence of unoccupied Lewis-acid sites on the particle
surface. In the presence of added triethylamine, however, the
octadecylphosphonate ligands are readily displaced from the particle
surface as was shown with 1H and {1H}31P NMR. These
results, in conjunction with previous literature reports, indicate that
as prepared nanocrystal surfaces are terminated by X-type binding of
octadecylphosphonate moieties to a layer of excess cadmium ions.

[0056] Although the foregoing invention has been described in some detail
by way of illustration and example for purposes of clarity of
understanding, one of skill in the art will appreciate that certain
changes and modifications may be practiced within the scope of the
appended claims.

[0057] Various features from the various embodiments may be combined in
any suitable manner without departing from the scope of the invention.

[0058] Any reference to "a," "an," or "the," is intended to mean "one or
more" unless specifically indicated to the contrary.

[0059] All references, patent applications, and patents noted above are
herein incorporated by reference in their entirety for all purposes. None
of the references, patent applications, or descriptions disclosed herein
is admitted to be prior art.